Forward viewing ultrasonic imaging catheter

ABSTRACT

A simple forward viewing ultrasound catheter includes one or more transducers and an ultrasound mirror supported by a bearing in a sealed end of a catheter with a drive cable imparting relative motion to the transducer and mirror. The mirror directs ultrasound waves forward of the catheter. An optical fiber can be provided to direct a laser beam for ablation of atheroma while under guidance of simultaneous intravascular ultrasound. A portion of a cavity can be simultaneously imaged while an interventional device operates on the imaged portion of the cavity.

This patent application is a continuation-in-part of copendingapplication Ser. No. 08/309,540 filed Sep. 19, 1994.

BACKGROUND OF THE INVENTION

This invention relates generally to vascular and other cavity imaging,and more particularly the invention relates to forward viewingultrasound imaging in vascular or other cavity treatment in which acatheter or other instrument can travel.

Coronary artery disease is the number one cause of mortality in theUnited States and peripheral vascular disease remains a major cause ofmorbidity. Percutaneous interventions have rapidly developed to addressthe blockages in the blood vessels which result in angina, heart attacksand limb ischemia. In 1990 greater than 300,000 coronary angioplastieswere performed in the United States. The methods for addressing theseblockages include balloon angioplasty as well as many newer technologiessuch as excimer lasers, directional coronary atherectomy and high speedrotational burrs. The traditional and still primary method for guidingthese interventions in angiography. Angiography is limited to definingthe size and course of the lumen of the blood vessel and therefore giveslittle information about the actual structure and geometry of theblockage and the blood vessel wall. Because of this limited imageguidance and primitive intervention devices, the incidence of acutecomplications remains significant, with a 3 to 5% rate of myocardialinfarction and 1 to 2% death rate. More importantly, the lack ofadequate visualization results in inadequate removal of the blockage andmay contribute to the high rate of recurrence.

Newer methods of visualization of the blood vessel have become availablein the past few years. Angioscopy allows visualization of the opticalcharacteristics of the surface of the blockage but gives no informationabout the underlying shape and structure of the blockage. Furthermore,angioscopy requires large amounts of flushing to keep the field of viewclear. Thus, angioscopy remains a poor method for guiding intervention.

Intravascular ultrasound has many of the properties of an ideal systemfor evaluating blockages and other lesions in blood vessels. Thecreation of images based on echo delay times results in visualization ofthe internal structure of the blockages and other lesions in bloodvessels. The creation of images based on echo delay times results invisualization of the internal structure of the blockage and of thearterial wall. Furthermore, since blood is relatively echolucent, noflushing is required to maintain an image, therefore continuous imagingduring intervention is feasible.

The current generation of intravascular ultrasound devices are allessentially side looking devices. As such, the device must be passedthrough the blockage in order for it to visualize the blockage. Sincethe smallest of the current generation of devices is 2.9 Fr (1 mm indiameter), the ultrasound catheter usually cannot be advanced through asignificant blockage without disturbing it. In the case of completeocclusions, the ultrasound catheter cannot be used at all.

A forward looking ultrasound device, that is a device which is notrestricted to side looking, would permit the evaluation of blockageswithout disturbing them and potentially serve as a useful tool forguiding recanalization of complete occlusions. The need for such adevice has been discussed for many years. Some degree of forward imaginghas been proposed in the past by angling the mirror used to redirect theultrasound beam so that a conical section is obtained, rather than theradial slice that results from a typical side looking transducer. Theconical sections obtained by this approach are not well suited forassessing the degree of atherosclerosis or for assessing the size of thelumen.

An implementation of a true forward viewing sector scanner was recentlydescribed which uses a complex mechanical linkage to achieve the forwardscanning. The complexity of this approach, however, has resulted in abulky device which measures 4 mm (14 Fr) in diameter. A device of thisdimension, although possibly suitable for use in the peripheralvasculature, could not be used in the coronary circulation.

In order to achieve the goal of a catheter suitable for evaluatingcoronary arteries as well as peripheral vessels, the device dimensionsshould be such that it will fit comfortably in a vessel 3 mm indiameter. Therefore, the catheter diameter should be less than 2 mm andideally under 1.5 mm. Furthermore, to provide useful images, the devicewill ideally provide more than 1 cm of penetration to permit completevisualization of most blockages and provide at least a 50 degree scansector so that the scan will subtend a typical 3-5 mm diameter vessel.

The present invention is directed to a mechanical sector scanner forachieving these goals. Although the features of this device are ideallysuited to work within blood vessels, it will also find value whereverultrasound imaging is needed in any cavity. These may be naturallypresent cavities or cavities created for therapeutic or diagnosticpurposes.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, an intravascularultrasound imaging catheter includes an ultrasound transducer and anultrasound reflective mirror supported by bearing means in a distal endof the catheter. A drive means such as a cable imparts relative motionto the transducer and mirror whereby ultrasonic waves are transmittedforward of the sealed end to provide blood vessel imaging. An opticalfiber can be provided to direct a laser beam for ablation of atheromawhile under the guidance of simultaneous intravascular ultrasound.

In specific embodiments, the transducer can be held stationary while themirror is rotated, the mirror can be held stationary while thetransducer is rotated, or relative motion can be imparted whereby boththe transducer and the mirror are rotated by means of planetary gears inthe bearing means. Further, a plurality of stationary transducers can bepositioned around the mirror and selectively energized as the mirror isrotated.

In accordance with another embodiment of the invention, the ultrasoundtransducer is rotatably or pivotally mounted in a housing fortransmitting a scanned ultrasound beam in response to a suitablemicroactuator driving mechanism such as an electrostatic orelectromagnetic responsive element.

A feature of the invention is an ultrasound imaging catheter which cansimultaneously image a vessel or other cavity while operating on theimaged portion of the vessel or cavity.

The invention and objects and features thereof will be more readilyapparent from the following description and appended claims when takenwith the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B illustrate in perspective and in section view anintravascular ultrasound imaging catheter in accordance with oneembodiment of the invention.

FIG. 2 illustrates scan path for the forward viewing catheter of FIG. 1.

FIG. 3 illustrates scan path with a variety of mirror orientations.

FIG. 4 illustrates scan angle resulting from mirror rotation for avariety of mirror orientations.

FIG. 5 illustrates relative aperture as a function of ultrasound beamangle for various mirror orientations.

FIGS. 6A, 6B are a perspective view and a section view of a catheter inaccordance with another embodiment of the invention.

FIG. 7 illustrates scan sectors for the embodiment of FIG. 6.

FIG. 8 illustrate relative imaging aperture as a function of scan angle.

FIG. 9 is a perspective view of a multi-transducer imaging catheter inaccordance with another embodiment of the invention.

FIGS. 10A, 10B are a perspective view and an end view of an imagingcatheter including planetary gears for generating coordinated mirror andtransducer motion.

FIGS. 11, 12 are side views of imaging catheters which employ rotatableultrasound transducers in accordance with embodiments of the invention.

FIG. 13 is a perspective view of an imaging catheter including laserablation.

FIGS. 14-17 are images generated with an intravascular ultrasoundimaging catheter in accordance with the invention.

FIGS. 18A-18C are perspective, front and side views of a multitransducerforwarding imaging device in accordance with the invention.

FIG. 19A and 19B are a side view and a section view of a multitransducerforward imaging device with a focused (parabolic) mirror.

FIGS. 20A, 20B are a side view and a cross-section view of a steerableimaging device in accordance with one embodiment of the invention, andFIGS. 20C and 20D are a side view and an end view of a steerable imagingdevice in accordance with another embodiment of the invention.

FIG. 21 is a perspective view of a steerable imaging device as in FIG.20 which includes a rotatable or linearly translatable imaging elementwithin a steerable sheath in accordance with an embodiment of theinvention.

FIGS. 22A and 22B are a side view and a perspective view of anotherembodiment of the invention in which a wire guide is maintained in thefield of view.

FIGS. 23A and 23B are a side view and an end view of another embodimentof the invention in which a therapeutic tool (cutting blade) is held ata fixed position in the field of view nd rotated with the mirror.

FIGS. 24A and 24B are a side view and an end view of another embodimentof the invention in which a therapeutic tool (laser) is positioned inparallel to the imaging device and maintained in the field of view ofthe ultrasound scan.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

FIGS. 1A and 1B are a perspective view and a side view of anintravascular ultrasound catheter in accordance with one embodiment ofthe invention in which an ultrasound transducer 10 is mounted on abearing 12 within a catheter housing 16 with a mirror 14 rotatable inthe bearing by means of drive cable 18. Transducer 10, a ceramic crystalfor example, is oriented so that its normal forms an angle τ with theaxis of the catheter, and the mirror, a polished stainless steel forexample, is oriented so that its normal forms an angle μ with the axisof the catheter, as shown in FIG. 13. The transducer bearing and mirrorare positioned in a sealed end of the catheter housing, which can be asuitable plastic tube, in a saline solution for ultrasound impedancematching with blood.

In writing the equation for the ultrasound beam direction, we define 3unit vectors, M the normal to the ultrasound reflectors, T the normal tothe ultrasound transducer and B the direction of the resultantultrasound beam. Given M and T, the direction of the ultrasound beam, B,will be given by:

    B=T-2(T·M)M                                       (1)

For convenience, the motion of the scanned beam can be described in acartesian coordinate system. The long axis of the catheter will bechosen so that it lies along the z-axis. The effect of angulation of themirror or the transducer with respect to the axis of the catheter by anangle α can be described by rotation of the normal vectors about thex-axis. This rotation is described mathematically by multiplication ofthe normal vectors by the following angulation transformation matrix,A(α). ##EQU1## The effect of rotation of the mirror or the transduceraround the long axis of the catheter by an angle α is described bymultiplying the normal to the mirror or the normal to the transducer,respectively, by a rotation transformation matrix, R(α). ##EQU2##

If we choose the reference mirror and transducer orientations such thatthe mirror faces forward along the axis of the catheter and thetransducer faces backward along the axis of the catheter, the normal tothe mirror after angulating the mirror by an angle μ and rotating by anangle θ and the normal to the transducer after angulating by an angle τand rotating by an angle φ are given by: ##EQU3## Given these formulasfor the normals to the mirror and the transducer, the resulting Beamdirection for arbitrary angulations and rotations of the mirror andtransducer can be calculated using Equation 1. The scan sectors forrotating mirror designs can be obtained by allowing the θ to vary from 0to 360 degrees. Similarly, the scan sector for rotating transducerdesigns can be calculated by allowing φ to vary from 0 to 360 degrees.

The applicability of this derivation can be seen by examining the resultwhen the mirror and transducer are in the orientations used for priorart side viewing devices. In this situation, the transducer is notangulated nor rotated, so τ=0 and φ=0. The mirror is angulated 45degrees, so μ=45°. M, the vector normal to the mirror, is given by:##EQU4## Simplifying: ##EQU5## Inserting this into equation 1, we get:##EQU6## Thus, we can see that the ultrasound beam with thisconfiguration of transducer and mirror traverses a circle in a clockwisefashion as the mirror is rotated 360° in the clockwise direction. Thescan path for a side viewing device is therefore a special case of thegeneral solution for an ultrasound scan produced by rotating a mirror ora transducer around the catheter axis.

A forward viewing device can be seen to result from another specialcase. If the transducer is now angulated 90° and the mirror angulated45°, the ultrasound beam vector will be given by: ##EQU7## Thisparticular arrangement of mirror and transducer can be implemented asshown schematically in FIG. 1. It consists of an ultrasonic transducermounted so that its beam is perpendicular to the long axis of thecatheter and positioned so that it strikes the face of a reflectingmirror which redirects the beam in a forward direction. Both thetransducer and reflecting mirror need be no different than thosecurrently used in side viewing ultrasound catheters, although thetransducer design should be thin in order to reduce the catheterprofile. Methods for constructing thin transducers are known. Rotationof the mirror can be by means of a flexible drive shaft as described formost side viewing catheters but may also use other means such asminiature motors or turbines.

The scan path created by such a device is shown graphically in FIG. 2.The sector scanned by this device lies approximately in the xz-plane,therefore the scan azimuth is defined to be the angle between the z-axisand the projection of the ultrasound beam onto the xz-plane. The degreeto which the beam deviates from being planar is defined by the scanelevation and is the angle between the ultrasound beam and the xz-plane.The formulas for the scan azimuth and scan angle are as follows:

    scan azimuth=arctan (sin θ)                          (11)

    scan elevation=arcsin (sin.sup.2 θ)                  (12)

The scan sector created by this simple device is thus minimally curvedfor a sector extending approximately 25°-30° to each side of midline,which gives a total sector width of 50°-60°.

The forward viewing device described above provides a 60° viewingsector. Outside of this range, the beam path begins to deviatesignificantly out of plane. Slight modifications of the transducer andmirror orientation can be used to improve the flatness of the scansector. It is still useful in these arrangements to force the ultrasoundbeam to point straight forward at some point during the scan. If weenforce this requirement, it can be shown that 2μ=τ, where μ and τ arethe angles that the normals to the mirror and to the transducer makewith the long axis of the catheter. Returning to equation 1 and nowsubstituting 2μ for τ, we get: ##EQU8##

The resulting scan path is then given by:

    scan angle=arctan (B.sub.x /B.sub.z)                       (14)

    scan azimuth=arcsin (B.sub.y)                              (15)

where B_(x), B_(y) and B_(z) are the components of the vector which isoriented along the direction of the ultrasound beam. A plot of thesectors scanned for a variety of mirror orientations is shown in FIG. 3.Each line represents a change of 5° in the orientation of the mirror.The flattest scan sector results from a mirror angle of ˜30° and atransducer angle of ˜60°. A gain of 5° in sector size is achieved byusing an oblique mirror and transducer angle when compared to the simpledesign presented in the previous section when using 10° as the acceptedlimit to which the beam can deviate from a planar scan. This improvementmay not be clinically important.

Since the scan azimuth is not equal to the angle of rotation of themirror, a correction will be needed prior to scan conversion to avoiddistortion. The correction is given in FIG. 4 for a variety of mirrororientations.

An additional issue that must be addressed is the reduction in imagingaperture as the mirror is rotated. As the mirror is rotated, theultrasound beam becomes progressively more oblique to the mirror whichwill result in loss of power received and transmitted, as well as someloss in resolution due to increased diffraction effects. If we assumethat the entire wave reflected off the mirror strikes the transducerduring receiving and conversely that the entire surface of the mirror isinsonified by the transducer during transmission, then the effectiveaperture of the device is given by (B·T)×mirror area. Referring back toequation 13, we get:

    Aperture area=(cos θ sin μ sin 2μ+cos μ cos 2μ)×mirror area                                  (16)

The change in aperture with mirror rotation is actually more relevantand can be obtained by dividing by the maximum aperture. The maximumaperture occurs when θ=0, therefore the maximum aperture achieved is cosμ×mirror area. The relative aperture size is then given by:

    Relative aperture area=2 cos θ sin.sup.2 μ+cos 2μ(17)

The resulting relative aperture as a function of the beam angle is shownin FIG. 5. It is apparent that some improvement in image aperture isachieved with a mirror pointed more coaxially with the catheter. Thegain is not large and by the point in the scan at which the aperturebegins to be compromised significantly, the scan will have alreadydeviated from the scan plane considerably. Within the useful scansector, the loss in aperture is only about 10% and should not besignificant.

Consider now the goal of achieving a planar scan sector. The forwardviewing devices described above do not achieve this ideal scan sectorbut produce a reasonable approximation for moderate sized scan sectors.The non-planar nature of the scan sector can be addressed by adopting amoving transducer design. A diagram illustrating one embodiment of amoving transducer device is shown in FIG. 6. Like elements in FIGS. 1and 6 have the same reference numerals. Strut 20 rigidly mounts mirror14 to bearing 12, and transducer 10 is free to rotate within bearing 12.If the transducer and mirror are oriented as they are for a rotatingmirror design and we continue to keep the constraint that the beam iscoaxial with the catheter at some point during the scan, we get for thebeam direction: ##EQU9## The resulting scan angle and scan can bedetermined from the components of B as before.

Of note when μ=45° then ##EQU10## It is easy to see that the ultrasoundbeam in a device of this design produces a beam that stays in thexz-plane and is oriented at angle -φ away from the axis of the catheterwhen the transducer has been rotated by angle φ. This would be the idealscan sector with linear correspondence between mirror rotation and beamangle and a large planar scan sector. The scan sectors resulting fromother mirror and transducer orientations are shown in FIG. 7. In generalfor all mirror orientations the scan sector remains much more planarwith a moving transducer design than with a moving mirror design.

The device will suffer from decreased aperture size as the beam becomesmore oblique to the surface of the mirror. It is easily shown that thedevice aperture is only dependent on the difference between the angle ofrotation of the mirror and transducer, thus we may use the result fromequation 17. The relative aperture as a function of scan azimuth isshown in FIG. 8. In contrast with the rotating mirror design which had ascan sector limited by deviation of the beam out of the scan plane, wesee that for mirror orientations near 45°, the limiting constraint onthe scan sector size is the loss of aperture when the beam is scanned tothe side. For scan sectors less than 90°, the loss in aperture is lessthan 30% and should be acceptable. A slightly shallower mirror angle canimprove the imaging aperture to some degree, the improvement, however,is slight for sector sizes up to 90° in width.

For a rotating transducer design and to a lesser degree for rotatingmirror designs, the overall catheter diameter will be significantlyaffected by the width of the transducer. For planar mirrors andtransducers, both the mirror and transducer must be of the samedimension to achieve maximal aperture, thus the effective aperture ofboth the rotating mirror and the rotating transducer designs device willnot be able to reach the theoretical maximum for a given catheter size,since a significant amount of space is needed to house the transducer.In particular, the moving transducer design requires more space toprovide clearance for the transducer as it rotates around the mirror. Ifa focusing mirror is used, the transducer may be made smaller withoutany change in the imaging aperture of the device and thus would permit adecrease in catheter diameter. Additionally, a smaller transducer isadvantageous for those designs where the transducer is oriented at lessthan 90°, since in those designs the transducer will impede part of thebeam reflected from the mirror unless it is set back from the mirror.

The scan of the sector is achieved in less than 120° of rotation of themirror for most choices of mirror and transducer angles. Therefore, mostof the duration of any rotation of the mirror will be spend in positionswhich are not useful for creating scan data. This scan time can berecovered by the addition of more transducers. For example, the use ofthree transducers 10 spaced equally around the catheter 16, as shown inFIG. 9, adds two additional scan sectors, each rotated 60° from theother. This approach may be applied to both a moving mirror or a movingtransducer design.

Three dimensional imaging is feasible for intravascular ultrasound atacceptable frame rates because of the relatively shallow depth ofpenetration needed. For a depth of penetration of 1.0 cm at a scan rateof 15 frames per second, a total of 5,000 A-lines may be obtained. Thiswill allow the forward field of view to be scanned by a 70 by 70 grid ofA-lines. The resulting scan line spacing closely approximates the beamwidth for achievable imaging apertures.

The multiscan approach can be extended to many transducers, thuscreating a multiplicity of sector planes. If enough planes are scanned,a three-dimensional image is produced. The number of wires required forsuch an approach may limit its suitability for generating C-scan(three-dimensional) imaging, although multiplexing schemes may beimplemented to reduce the number of wires required.

An alternative approach is to move the transducer after each sector isobtained. The transducer position may be rotated around the axis of thecatheter or displaced along the axis of the catheter. Rotation of thetransducer results in a series of sector scans which are graduallyrotated around the axis of the catheter. Displacement of the transducerresults in each scan sector being displaced in azimuth from theprevious. Either approach results in a three-dimensional scan. Boththese approaches have the disadvantage of requiring the coordinatedmotion of both the mirror and the transducer. One approach to generatingthe coordinated motion of the mirror and transducer is to drive thetransducer motion via a set of planetary gears 24, as shown in FIG. 10.The diameters of the gears are chosen to achieve the desired ratio oftransducer rotation to mirror rotation. Each rotation of the mirror willthen start with the transducer in a slightly different orientation andthus the sectors scanned by the mirror motion will offset in angle fromeach other.

A B-scan or C-scan requires the controlled deviation of the ultrasoundbeam away from the catheter axis. A prior art device uses a system ofmechanical cams to convert rotary motion into the proper angulation of atransducer. In accordance with the invention, the angulation is achievedby reflecting the ultrasound beam off a rotating mirror.

The forward viewing catheters described above use a mirror movingrelative to a transducer to achieve a forward viewing scan. Analternative approach is to dynamically tilt the transducer and thusdirectly redirect the ultrasound beam. This approach has been previouslyproposed using a system of cams and follower pins to convert the rotarymotion provided by the drive cable in to the necessary tilting motion.This mechanical approach however limits the size of the catheter becauseof its mechanical complexity.

A more direct approach is to use microactuators located at the tip ofthe catheter to directly generate the motion needed. The microactuatorscan be electrostatic, electromagnetic, or unimorphs or monomorphs basedon thermal or piezoelectric effects.

Alternatively, mechanical movement can be obviated by electronicallyswitching the transducer elements to steer an ultrasound beam.

One embodiment is shown in side-view in FIG. 11. In this embodiment thetransducer 10 is mounted on a shaft 40, including a unimorph ormonomorph actuator in cylindrical housing 16. The shaft is then passedthrough a hole in the tip of a hollow stainless steel cone 42,positioned in housing 16, and is supported at an opposing end by supportdisk 44. Wires 46 energize the unimorph or monomorph. The end of theshaft is then deviated from side to side with the unimorph or monomorphactuator. The cone acts as a fulcrum thus resulting in tilting of thetransducer from side to side. The use of two actuators mounted 90degrees from each other would give the potential to scan in twodirections and thus provide 3-dimensional imaging.

An alternative design is shown in FIG. 12. Here the transducer 10 ismounted in a echolucent hemisphere or sphere 44 with a rubber orabsorbent material as a backing for the transducer. Such echolucentmaterial is well-known, such as ATV-66. The sphere is free to rotate ina socket 49 at the tip of the catheter. A small hole 48 is placed in theback of the sphere into which the tip of a microactuator 40 is inserted.As the tip of the microactuator is deviated from side to side underelectronic control the sphere will be rotated, thus tilting thetransducer.

The imaging probe can be combined with a laser to allow ablation ofatheroma while under the guidance of simultaneous intravascularultrasound. One method for achieving this is to carry the laser energyto the catheter tip in optical fibers as shown in FIG. 13. At thecatheter tip, the light energy is allowed to leave the fiber 26 and isredirected by mirror 28 so that its path is coaxial with the beamgenerated by the ultrasound transducer. This polished stainless steelmirror is reflective to ultrasound waves and laser waves. The laser beamand the ultrasound beam are then scanned in unison by the rotatingmirror. This necessitates that the ultrasound mirror also function as anoptical mirror. By switching the laser on and off in correspondence withthe mirror rotation, a particular point in the atheroma as identified byultrasound may be selectively ablated.

To demonstrate the principle of this device, a 10 Fr and a 4.3 Frcommercial side viewing intravascular ultrasound catheter were modifiedto achieve the geometry shown in FIG. 1. The mirror angle was 45° andtransducer was oriented perpendicular to the catheter. This design wasused because of the simplicity of manufacture.

Both probes were driven with a commercial intravascular ultrasoundmachine (CVIS Insight). A pulse frequency of 10 MHz was used with the 10Fr probe and 30 MHz was used with the 4.3 Fr probe.

The images shown in FIGS. 14 and 15 were obtained with the 10 Fr probe.No correction for the less than one to one correspondence between themirror rotation and the scan angle was performed, therefore there issome distortion at the edges of the images. The actual images have beencropped to remove the portion of the scan time in which no usefulinformation is obtained.

Imaging has also been performed upon cadaver tissue. FIG. 16 wasobtained with a 4.3 Fr catheter operating at 30 MHz. The catheter wasplaced in the lumen of a freshly explanted human cadaver aorta. Thisvessel was without disease as is evidenced by the smooth vessel walls onthe ultrasound image. A cross section of a small vessel arising from theaorta is shown in FIG. 17. The lumen of the small vessel is quite easilyidentified.

Described above is the use of a relative rotational motion of anultrasound transducer and mirror to generate a forward viewing scan. Thegeneration of the relative rotations can be via a mechanical means suchas a flexible shaft or a micromotor. The relative rotation of theultrasound source and the mirror need not be by mechanical motion, anyone of many ways for moving the relative location of the ultrasoundsource may be used. For example, the orientation of the activetransducer can be changed by shifting from transducer to transducer orfrom groups of transducers to another group of transducers. Such adevice may be implemented as shown in FIGS. 18A-18C, which areperspective, front, and side views of a plurality of transducer elements50 positioned circumferentially around rotating mirror 52 withincatheter 54. The shifting of the activated transducer can beaccomplished by using a multiplexing circuit at the tip of the catheteror by having multiple signal cables traveling the length of thecatheter.

In order to generate a sector scan by electronically switching todifferent transducers, a sufficiently large number of transducers togenerate the required number of scan lines is needed. For a typicalcatheter with a 0.5 mm aperture the ultrasound beam width isapproximately 6 degrees. Since the deviation of the ultrasound beamapproximates the deviation of the position of the transducer around thecatheter, the transducer elements should be spaced at about 6 degreespacings. This requires that the transducer elements be small. If a flatmirror is used, the resulting imaging aperture will be small, limitingthe resolution of the device. The use of a focused mirror allows theeffective imaging aperture to approximate the diameter of the mirror.The design for a device using switching of transducer elements and afocused mirror is shown in the side view and section view of FIGS. 19Aand 19B.

The shifting of the transducer elements will bring the active transducerelement closer to the mirror. This will result in the focal point of theresultant beam moving further away from the device tip. For a typical60° sector width, the resultant increase in depth of focus as the mirrorrotates is only 30%. The effect of this shift in focus should benegligible within the central portion of the scan.

In FIG. 19 the curvature of the mirror face should be approximatelyparabolic with vertex to parabola focus length of: ##EQU11## whered=distance from transducer to center of catheter and fo=distance frommiddle of mirror to desired focal point of ultrasound beam.

An alternative approach to using a focused mirror is to use phased arrayor synthetic aperture approaches to generate high resolution scanselectronically as described in A. Macovski, "Medical Imaging Systems",chapter 10, pages 204-224, Prentice-Hall, Inc. 1983. In the phased arrayapproach, the individual elements of the array are delayed in theiractivation to generate a planar wave front; approximating the excitationthat would be created by a single large flat transducer. Duringreception, the signals from each of the elements are delayedappropriately to account for the difference in distance between themirror and the different transducer elements; again approximating theresponse of a single large flat transducer. The scan is generated bychanging the timings of the various delays to generate wavefronts atslightly different angles.

The difficulty with this approach is the requirement for a large numberof wires within the body of the catheter. A synthetic aperture approachreduces the need for multiple wires by recording the echoes from each ofthe transducers sequentially from a sequence of repeated pulses. Sinceechoes for each of the pulses are substantially the same over theduration of the pulse train, the signals from each of the transducerscan still be combined with the appropriate delays to localize the originof the echo. If an omnidirectional transmit pulse is used, the totalnumber of pulses needed to generate the image is not substantiallychanged since the same data set of echoes can be combined with slightlydifferent delays to obtain information from different locations.

The use of omnidirectional pulses results in a reduction of the angularresolution since the excitation is no longer spatially localized. Thiscan be corrected by using each of the transducer elements in sequenceand generating a synthetic transmit aperture. This approach requiresthat n² pulses be transmitted for each image sector, where n is thenumber of elements in the transducer array. Approaches using sparsearray techniques can be applied to reduce the number of pulses, asdisclosed in G. R. Lockwood, Pai-Chi Li, M. O'Donell, and F. S. Foster,"Optimizing the Radiation Pattern of Sparse Periodic Linear Arrays",IEEE Trans. on Ultrasonics, Ferroelectrics and Frequency, 43:7-14,January 1996.

The combination of mechanical and electrical scanning can be combined togenerate true 3-D scanning. To generate true 3-D scanning using entirelymechanical means requires the mirror and the transducer to beindependently rotated around the axis of the catheter. This can beaccomplished by either using a gear or clutch mechanism at the tip ofthe catheter to allow the transducer and mirror to move at differentangular velocities using a single drive cable, as was discussed in theoriginal patent application. Alternatively, two independent drivemechanisms can be used. These could be two cables, two micromotors or acombination of a drive cable and a micromotor. All of these approachesare however limited by their mechanical complexity. If, however, thechange in transducer position is generated by switching from onetransducer element to another as discussed in the above section, themechanical design is simplified considerably. Furthermore, sinceelectrical switching can be performed more quickly, the higher scanrates needed to obtain information from the large number of planes usedin generating a 3-dimensional scan can be more easily accommodated.

The ability to see forward with this catheter creates the possibility ofimage guided advancement of this catheter rather than the traditionalwire guided catheter positioning. This possibility makes the ability tosteer/deflect the catheter much more valuable. There are a variety ofmethods for steering/deflecting a catheter tip.

The simplest is to use a system of cables 58 passing along the length ofthe catheter 60 to deflect the catheter by generating strain localizedto one side of the catheter, as shown in FIGS. 20A and 20B.

An alternative means of generating differential strain is to use anelement 62 which deforms in response to changes in temperature orelectrical potential as shown in FIGS. 20C and 20D. These bimorphelements may be shape memory alloys, piezoelectric films or conductivepolymers. The advantage of these means is that deformation of the shaftof the catheter should have no effect on the motion of the tip.Construction of the catheter may be simplified since fewer mechanicalelements need to traverse the length of the catheter.

In certain situations, it is desirable to advance and/or rotate thedevice while maintaining its general direction. When this is desirable,a design which incorporates the steering elements in a sheath 64 throughwhich the device 66 is passed becomes desirable, as depicted in FIG. 21.The steerable sheath may be implemented using the same approachesoutlined for making a steerable catheter.

A forward viewing intravascular ultrasound will have value in guidinginterventions. A forward viewing ultrasound catheter is unique in thatit permits imaging of the site of intervention as the intervention isbeing performed an thus provides continuous feedback on the nature ofthe material being treated with the interventional device. Priordesigns, as disclosed in U.S. Pat. Nos. 4,794,931 and 5,000,185, forcombined ultrasound and interventional devices have not permittedimaging of the site of intervention while the intervention is inprogress. The small size of the vessels and the need for coordinatedaction of the interventional device and the imaging catheter are issuesthat must be addressed. The following are embodiments of the inventiondirected to these issues.

One of the primary uses of a forward imaging catheter will be in theguidance of recanalization of totally occluded vessels and to predictthe path of an interventional device by providing a continuous view ofthe internal device. To accomplish this, it is desirable to keep theguide wire or other recanalization tool in the field of view. This issueis particularly important when a 2-dimensional scanner is used. Toaccomplish this, a wire guide 68 with a guide wire or laser wire 70 maybe incorporated into the imaging catheter 72 directly ahead of thecatheter to ensure that the guide wire enters the imaging plane, shownin the side view and perspective view of FIGS. 22A and 22B.

The ultimate goal of image guided therapy may be best approached with acombined imaging and therapy device where the therapeutic tool is heldat a fixed position in the field of view of the imaging catheter. Thewhole device is then steered in the fashion previously described. Thetherapeutic device may be a laser catheter, electro-erosion, rotationalburr or a cutting device. Where the therapeutic device uses or cantolerate rotational motion, it may be spun with the drive means used todrive the imaging scan. When the acceptable rotational frequencies ofthe rotational device matches those of the imaging scanner, thisapproach eliminates the need for a second drive cable, thus reducing thecomplexity and size of the device.

FIGS. 23A and 23B are a side view and an end view illustrating theincorporation of a cutting device 74 into the imaging device 76. Thecutter is attached to the mirror 76 and spins with the mirror 76.Transducer 78 is shown mounted on catheter housing 80. Two cuttingblades on the tip of device 74 are designed to shave bits of atheromaoff the occlusion. The debris is aspirated back through the hollow coreof the mirror and cutter drive shaft. Cutting blade apertures aredesigned to produce debris small enough so that it may pass through thehollow core.

Where the therapeutic device cannot be spun or must be spun at speedsincompatible with the ultrasonic imaging, the device can be positionedin parallel to the imaging device, as shown in FIGS. 24A and 24B where alaser angioplasty device 82 runs parallel to the mirror 76 withincatheter 80. The therapeutic device is then oriented so that the pointof contact of the device with atheroma is in the field of view of theultrasound scan.

An alternative approach to maintaining coordinated motion of the imagingand the therapeutic device is to limit the independent motion of thetherapeutic device to those motions which keep the device in the planeof imaging. In the case of a two dimensional imaging device, this isaccomplished by allowing active or passive deflection of the device inthe plane of imaging. Addressing areas of plaque outside the plane ofimaging is accomplished by rotating the plane of imaging.

Designs for a simple and compact forward viewing catheter and severalapplications thereof have been described. Design considerations foroptimal scan trajectory and aperture are reviewed, and improving thescan rate using multiple transducers is described. Implementingtwo-dimensional scanning to collect real time three-dimensional datasetsis also described.

While the invention has been described with reference to specificembodiments, the description is illustrative of the invention and is notto be construed as limiting the invention. As noted above, theultrasound imaging catheter can be employed in any cavity whereultrasound imaging is needed. Thus, various modifications andapplications might occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. An ultrasound imaging catheter comprisingacatheter having a distal end and a longitudinal axis, ultrasound meansmounted within said catheter and spaced from said longitudinal axis forgenerating an ultrasound beam at an angle to said longitudinal axis, amirror mounted within said catheter adjacent to the distal end forredirecting said ultrasound beam in a forward direction, and means forimparting relative rotational orientation between said ultrasound meansand said mirror whereby said ultrasound beam is redirected generally ina pattern extending forward of said distal end, said relative rotationalorientation being about an axis extending in a generally forwarddirection.
 2. The ultrasound imaging catheter as defined by claim 1,wherein said ultrasound means comprises an array of ultrasoundtransducers positioned circumferentially within said catheter aroundsaid mirror.
 3. The ultrasound imaging catheter as defined by claim 2,wherein said mirror has a curved surface to generate focusing of anultrasound beam and permit the use of smaller ultrasound transducers. 4.The ultrasound imaging catheter as defined by claim 2, wherein relativerotational orientation is achieved by varying signal transmit and signalreceive delays of said transducers.
 5. The ultrasound imaging catheteras defined by claim 4, wherein responses for each transducer areobtained from sequential transmit pulses and are recombinedmathematically to generate an effective planar transmitter and receiver.6. The ultrasound imaging catheter as defined by claim 1, whereinthree-dimensional scanning is accomplished by altering orientation ofboth said mirror and said ultrasound means.
 7. The ultrasound imagingcatheter as defined by claim 6, wherein responses for each transducerare obtained from sequential transmit pulses and are recombinedmathematically to generate an effective planar transmitter and receiver.8. The ultrasound imaging catheter as defined by claim 6, whereinalteration of at least one of said mirror and said ultrasound means iseffected by a micromotor.
 9. The ultrasound imaging catheter as definedby claim 1 and further including a mechanism to reorient a tip at saiddistal end.
 10. The ultrasound imaging catheter as defined by claim 9,wherein said mechanism comprises cables located eccentrically withinsaid catheter.
 11. The ultrasound imaging catheter as defined by claim9, wherein said mechanism comprises a shape memory alloy mounted nearthe tip of said catheter.
 12. The ultrasound imaging catheter as definedby claim 9, wherein said mechanism comprises at least one of a bimorphand a unimorph component mounted near the top of said catheter.
 13. Theultrasound imaging catheter as defined by claim 9, wherein saidmechanism comprises a deformable conductive polymer component mountednear the tip of said catheter.
 14. The ultrasound imaging catheter asdefined by claim 1, wherein said ultrasound means and said mirror aremounted within said catheter which is linearly translatable within adeflectable sheath.
 15. The ultrasound imaging catheter as defined byclaim 1 and further including a steerable guide at said distal end ofsaid catheter for directing an interventional device into a field ofview of said ultrasound imaging catheter.
 16. The ultrasound imagingcatheter as defined by claim 1 and further including a device forremoving plaque mounted proximally to said imaging catheter whereby saiddevice remains at a fixed point in the field of view.
 17. The ultrasoundimaging catheter as defined by claim 16, wherein said device comprises afiber optic guide for the delivery of laser energy.
 18. The ultrasoundimaging catheter as defined by claim 16, wherein said device comprisesan electrode for the delivery of electrical energy for electro-erosionof plaque.
 19. The ultrasound imaging catheter as defined by claim 16,wherein said device comprises a rotatable abrasive device.
 20. Theultrasound imaging catheter as defined by claim 16, wherein said devicecomprises a cutting head on a central lumen for the aspiration ofremoved plaque.
 21. The ultrasound imaging catheter as defined by claim16, wherein said device is coaxial with said imaging catheter, saiddevice protruding through the center of said mirror and remaining in thefield of view.
 22. The ultrasound imaging catheter as defined by claim21, wherein rotational motion of said mirror drives said device.
 23. Theultrasound imaging catheter as defined by claim 22, wherein said devicecomprises a cutting head.
 24. The ultrasound imaging catheter as definedby claim 22, wherein said device comprises an abrasive device.
 25. Theultrasound imaging catheter as defined by claim 1 and including aninterventional device which is steerable within the field of view ofsaid ultrasound imaging catheter.
 26. The ultrasound imaging catheter asdefined by claim 1 and further including an interventional device andwherein said catheter simultaneously images a portion of a cavity whilesaid interventional device operates on the imaged portion of saidcavity.